1. Field of the Invention
The invention relates generally to noise rejection in interferometric displacement measurements.
2. Background Art
Interferometry is a well known technique for measuring the phase difference between two or more optical beams. Two-beam interferometers, where one of the optical beams is back reflected by an object surface and the other beam is used as a reference, are used to monitor small deformations on an object under test or small displacements of a surface of a workpiece under test.
Multi-speckle interferometers are able to collect a larger amount of light compared to single-speckle interferometers. Increase in the amount of collected light leads to an increase of the interferometer sensitivity. Multi-speckle interferometry may be performed using, for example, confocal Fabry-Perot or multi-channel quadrature interferometers, or two-wave mixing in a photorefractive crystal. A Michelson interferometer is a typical single-speckle interferometer.
Multi-speckle interferometers may be used to detect displacements of an object that is subjected to ultrasound. Laser ultrasonics can advantageously be used for nondestructive testing in order to measure the thickness of objects or to monitor defects in materials. Industrial applications involve the inspection of an optically rough surface, leading to the ultrasonic information being encoded in a laser beam with speckles.
Like every detection system, multi-speckle interferometers are subjected to noise, i.e., electronic noise, shot noise, laser phase noise, and laser intensity noise.
The phase noise is related to the coherence of the laser and needs to be considered when the path difference between the interfering beams is no longer small compared to the coherence length of the laser. For dual-beam interferometers, the coherence length requirement is not critical since the signal and reference beam paths can be made nearly equal. With current laser technology, laser coherence lengths of several meters are commonly available, thus reducing phase noise considerably.
Electronic noise adds an unwanted characteristic of an electronic circuit to the wanted signal. The electronic noise may be considered independent of the intensity of the light detected by a detector. It may, for example, only depend on the first amplifier stage used to amplify a weak photo-detector current. In a well-designed system, the electronic noise should only amount to a small fraction of the total noise.
The shot noise is the quantum noise due the light-to-electrical conversion in the detector, such as a photodiode. In the case of a coherent light source, shot noise increases with the square root of the intensity arriving on the detector.
Laser intensity noise usually results from vibrations of the laser cavity or fluctuations in the gain medium of the laser. Typically, intensity noise is proportional to the laser intensity. At ultrasound frequencies below a few MHz, the intensity noise can quickly become the limiting factor for the interferometer sensitivity. It is possible to use specific lasers with low intensity noise, but these are expensive.
It is an aim of the invention to reject the laser intensity noise such that the shot noise is the dominant noise source in the system. This way, the interferometric system may achieve its theoretical sensitivity limit without using specific lasers.
One method of rejecting intensity noise is balanced detection, as described, for example, in C. B. Scruby and L. E. Drain, “Laser Ultrasonics—Techniques and Applications”, Ed. Adam Hilger, Bristol, UK (1990). In a detection-balanced interferometer, two interference beams of equal intensity and opposite phases, both carrying the surface information from the object, are detected using two photo-detectors. The two electrical signals corresponding to the interference beams are then subtracted so that the phase information is summed up and the intensity noise is cancelled out. However, balanced detection is more difficult and expensive to implement since the optical set-up necessary to properly align the intensity and the phase of the two interference beams is more complex.
Another method to reject intensity noise is an all-electronic method as described, for example, in Philip C. D. Hobbs, “Ultrasensitive laser measurements without tears”, Applied Optics, Vol. 36, No. 4, February 1997 (also in U.S. Pat. No. 5,134,276). In an all-electronic scheme, photocurrents generated by an interference beam and a comparison beam sampled at the laser output are processed. The two photocurrents are combined to obtain an output signal where intensity noise has been rejected. The combination may be provided through subtraction or division of the photocurrents. In both cases, the comparison beam draws off intensity from the total laser output intensity that can hence not be used for the interferometric measurement.
The invention aims at providing an interferometer that is able to reject laser intensity noise without increasing shot noise, and, at the same time, to use as much of the available laser light as possible to obtain the highest possible sensitivity of the interferometric system.